1. Field of the Invention
The present invention relates to a phase-change optical recording media in which reversible change of atomic arrangement in a recording film between crystalline and amorphous phases is brought about by irradiating the recording film with a light beam so as to record information.
2. Description of the Related Art
(Principle of Phase-Change Optical Recording Media)
A phase-change optical recording media, which uses a phase-change optical recording film that causes a reversible phase-change between crystalline and amorphous phases on irradiation with a light beam, operates according to the following principle. A write operation is performed by heating the region irradiated with a light beam to temperatures higher than the melting point of the film to melt that region, followed by rapidly cooling to make atomic arrangement in that region amorphous. An erase operation is performed by retaining for at least a prescribed time the region irradiated with the light beam within a temperature range from the crystallization temperature or more to the melting point or less. In this operation, if the region is crystalline in the initial state, it remains crystalline, and if the region is amorphous in the initial state, it is crystallized (solid-phase erasing mode). Depending on a material for the recording film, a method of heating the vicinity of an amorphous region in the recording film to the melting point or more to melt that region and then slowly cooling and crystallizing that region (melt erasing mode) may be adopted. A read operation (reproducing) is performed by converting the intensity of a reflected beam into the intensity of an electric signal utilizing the phenomenon that the intensity of a beam reflected from an amorphous region is different from that of a beam reflected from a crystalline region, followed by subjecting the electric signal to analog-to-digital (A/D) conversion.
It should noted that write and read operations can be performed by utilizing a transition between a metastable crystalline phase and a stable crystalline phase as in martensite or a transition between metastable crystalline phases as well as the phase-change between the crystalline and amorphous phases noted above.
(Methods for Increasing Density)
Two methods described below may be used to increase the amount of information that can be recorded in a single recording media, i.e., the recording capacity.
One of the methods is to reduce the pitch of recording marks in the track direction. However, when the pitch of recording marks is significantly reduced, it reaches a level that is smaller than the size of a read beam. In such a case, two recording marks may be temporarily included in a read beam spot. If the recording marks are sufficiently separated from each other, a read signal is significantly modulated to have high amplitude. However, if the recording marks are close to each other, the signal has low amplitude, with the result that an error is easily generated in converting the signal into digital data.
The other method is to reduce the track pitch. This method enables to increase the recording density without being significantly subjected to degradation of signal intensity, unlike the case of reducing the mark pitch. However, a problem with this method is that, in such geometry that the track pitch is equal to or smaller than the size of a light beam, a so-called cross-erase may be caused in which the information in a certain track is degraded when a write or erase operation is performed on the adjacent track.
The possible causes of the cross-erase are as follows. First, the recording mark on a certain track is directly irradiated with the outer peripheral portion of a laser beam applied to the adjacent track. Second, heat generated in the write operation to the adjacent track flows into the track in question to raise the temperature of the mark on the particular track and to deform the mark. These problems must be solved in order to increase the density of the phase-change optical recording media. Also, in order to suppress probability of read error for small recording marks to a low level, it is desirable that the recording marks be formed in a manner to have a smooth contour so as to suppress a noise component as much as possible.
(Increase in Capacity by Using a Multi-layer Media)
Another technique for increasing the capacity involves stacking a plurality of information layers each including a phase-change optical recording film (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2000-322770). The media in which two information layers are stacked so that a read or write operation can be performed on one side of the media is called a single-sided, dual-layer media or simply a dual-layer media. Two single-sided, dual-layer media may further be stacked to obtain double-sided, quadruple-layer media to further increase the capacity. In the single-sided, dual-layer media, a first information layer (referred to as L0 hereinafter) closer to the light incident plane must have a transmittance of at least about 50%. This is because it becomes important to prevent light from being attenuated in the L0 layer more markedly than required, when a second information layer (referred to as L1 hereinafter) which is remote from the light incident plane is accessed. To achieve this, the phase-change optical recording film in the L0 layer must be as very thin as 10 nm or less. Such a thin phase-change optical recording film increases the retention time required for crystallization, resulting in generation of non-erased bits, i.e., decrease in an erase rate, at a normal write speed.
As one of the measures for this problem, it is known to be effective to substitute Sn for a part of a GeSbTe recording film (see Proceedings of the 12th Symposium on Phase-Change Optical Information Storage PCOS 2000, pp. 36-41). Similarly, it is known to be effective to substitute Bi, In, Sn, or Pb for a part of the GeSbTe recording film (see Jpn. Pat. Appln. KOKAI Publication No. 2001-232941). However, such improvement as modification in the composition of the recording film is insufficient to compensate for a reduced crystallization speed accompanied by decrease in the thickness of the recording film. Accordingly, it has been proposed to provide, for example, a germanium nitride (GeN) film which serves as an interface film effective for accelerating crystallization at an interface with the recording film (see Proceedings of the 12th Symposium on Phase-Change Optical Information Storage PCOS 2000, pp. 36-41, mentioned above). However, it has been found from studies made by the present inventors that cross-erase occurs with the combination of a thin recording film of 10 nm or less and a conventional interface film such as GeN, making it impossible to reduce the track pitch effectively. Further, it has been found that the use of silicon carbide (SiC) which has been reported to exhibit a function of accelerating crystallization as an interface film results in a great extinction coefficient at the wavelength of 405 nm for a blue-violet laser (LD) used in next-generation high-density optical disks, leading to a very heavy optical loss. It has also been found that an interface film formed of germanium nitride (GeN) or silicon nitride (SiNx) brings about an optical loss.
On the other hand, a media without the interface film can suppress crystallization of a melted portion to minimize the occurrence of cross-erase. However, such a media has been found to have a very insufficient erase rate. Further, in the L1 layer, a write or erase operation must be performed using a laser beam having an intensity reduced to half as a result of passage through the L0 layer. This requires the sensitivity of the media to be increased. Therefore, it is also important to reduce the optical loss in the interface film or dielectric film in order to increase the availability of a laser beam.
(Method for High-speed Recording)
High-speed recording is another requirement for phase-change optical recording. When a movie is being recorded, for example, if the recording can be completed in a time shorter than the actual viewing time, it is possible to easily accomplish a so-called time shift function that enables an audience to view previous video during dubbing of a distributed media or during broadcast recording. Here, one of factors hindering a high-speed operation in phase-change recording is a problem of an insufficient erase rate. That is, when an erase beam with a relatively low power level is used for crystallization during overwriting, a certain amount of information may remain without being erased. This problem occurs because a recording mark passes through a laser spot at high speed, and thus the recording mark cannot be retained for a sufficient time under a temperature range within which crystallization is enabled, so that a certain amount of information may remain.
As an improvement to facilitate crystallization so as to increase erase speed, it has been disclosed to provide an interface film formed of a material such as GeN in contact with the recording film (see Jpn. Pat. Appln. KOKAI Publication No. 11-213446). However, when making experiments using the material disclosed in this document as an interface film, the inventors found that a melted portion is partly recrystallized during recording, which means that a larger area must be melted in order to produce a recording mark of a required size. Since the use of such an interface film leads melt of an area larger than necessary, the occurrence of cross-erase is facilitated, which produces adverse effects in view of high-density recording. In other words, when recording is performed with a laser power within a range permitted in view of cross-erase, the width of a recording mark formed is reduced, leading to a problem of reduction in carrier-to-noise ratio (CNR). On the other hand, it has been found that a media without any interface film can suppress recrystallization of the melted portion and thus can suppress cross-erase, but provides a very insufficient erase rate. Therefore, a novel interface film which can suppress recrystallization of a melted portion during recording while increasing the crystallization speed in erasing is demanded.
(Film Design for Phase-change Optical Recording Media)
With phase-change optical recording media, as previously described, an amorphous mark which is data is written in a desired portion of the recording film by irradiation with a laser pulse, and data is erased by irradiating the amorphous mark with a laser beam so as to crystallize the mark. In the former operation, the amorphous mark is formed by rapidly cooling the portion irradiated with the laser beam. In the latter operation, the amorphous portion is crystallized by slowly cooling the portion irradiated with the laser beam. Further, if the recording film has a high absorbance, a write or erase operation can be performed at a low laser power. Conversely, if the recording film has a low absorbance, a high laser power is required to perform a write or erase operation. The absorbance of the recording film is determined by the optical characteristics of the media formed of a multilayer film. Furthermore, even with similar absorbance, the media can be a rapid or slow cooling structure or can produce anisotropy of thermal characteristics between the in-plane direction and the sectional direction.
That is, optical and thermal designs are mainly considered for the film design for the phase-change optical recording media. The optical design requires the optical characteristics of each thin film. The thermal design requires thermal characteristics including the melting point, latent heat of melting, and crystallization temperature of each thin film. The optical constants of a thin film can be measured using an ellipsometer. Whereas, several studies have indicated that the thermal characteristics of thin films of the order of nanometers are different from those of a bulk. However, it has been impossible to systematically measure the thermal characteristics of thin films while eliminating the influences of other factors. Thus, empirical parameters have been required to correct measurements of thermal characteristics of thin films. In particular, there have been almost no methods for measuring the interface heat resistance between thin films of the order of nanometers.
(Interface Film Material)
As materials which has a function of accelerating crystallization and can be used as interface films, other than GeN, materials comprising an oxide such as Ta2O5 into which a carbide or a nitride is mixed have been disclosed (see Jpn. Pat. Appln. KOKAI Publication No. 2003-67974). The oxide such as Ta2O5 is intended to be used as sulfur-free protective film material. These materials have been examined in order to principally improve current DVDs using a laser with a wavelength λ of 650 nm. However, these materials are opaque at a wavelength λ of 405 nm for next-generation blue-violet LDs, and incur a heavy optical loss. Thus, the materials are inappropriate for use in next-generation high-density media. Further, as described above, GeN, initially proposed as the interface film, is also opaque and incurs a heavy optical loss at the wavelength of next-generation blue-violet LDs. Thus, the currently disclosed techniques do not provide any interface film materials which are optically transparent at the wavelength of blue-violet LDs and which provide a function of accelerating crystallization.
On the other hand, it has been reported that a cap layer such as AlOxNy, HfOxNy, Si3N4 and In—SnOx used together with a so-called eutectic recording film can effectively improve overwriting (OW) characteristics (A. E. T. Kuiper et al., Applied Physics Letters, Vol. 82 (2003), p. 1383). The arrangement of the cap layer is almost similar to that of the interface film. However, the recording film used in this document is formed of a eutectic material and uses a technique for erasing data by applying a laser beam to a portion where the data has been written thereby melting that portion (so-called melt erase). Accordingly, the cap layer is provided to suppress the diffusion of sulfur (S) in ZnS—SiO2, serving as a protective film, into the recording film. Further, this document discloses only the data on In—SnOx (so-called ITO), and does not disclose whether other materials can suppress the diffusion of sulfur (S). Furthermore, although the document discloses data indicating that the cap layer improves the OW characteristics, the document discusses the data on the cap layer consisting of SiC and does not disclose any data on the aforementioned materials that would be effectively used as a cap layer. Thus, the document only suggests that the optimum material be selected for the cap layer. Moreover, industrial applications require not only appropriate material and composition of the cap layer but also the detailed description of manufacturing conditions. Therefore, the document does not disclose any completed techniques.
As described above, Sic is known to exhibit a higher absorbance than the GeN or Ta2O5-based material at the wavelength λ of 405 nm for blue-violet LDs used for next-generation optical disks. Thus, if SiC is used as an interface film in an optical disk using a blue-violet LD, it would lower the sensitivity. In addition, if SiC is used as an interface film in the L0 layer of a single-sided dual-layer media, it would lead to decrease in transmittance.
(Material System for Recording Film)
As described above, since the eutectic recording film uses the melt erase mode, the cap layer to the eutectic recording film is not expected to provide the crystallization acceleration function. Thus, the details of the film material and structure thereof have not been examined. Further, since the eutectic recording film uses the melt erase mode, it is very difficult to carry out so-called land/groove recording in which information is written to and read from both land (L) and groove (G). This is very disadvantageous for an increase in recording density.
On the other hand, a so-called pseudo-binary recording film material such as Ge2Sb2Te5 can cause phase change from amorphous state to crystalline state at a high-speed in a solid state (solid-phase erase mode), without using the melt erase mode. However, a thin recording film requires a relatively long time for crystallization. Consequently, it is essential to use an interface film having a crystallization acceleration function by which the land/groove recording can be realized.
A data erase process carried out with a eutectic recording film is completely different from that carried out with a recording film of a pseudo-binary system in terms of phenomenon. Thus, the properties required for the cap layer are different from the functions required for the interface film represented by the crystallization acceleration function. Therefore, in order to find a suitable interface film material, it is necessary not only to select an appropriate film material but also to examine the structure and composition of the film material in detail.